How does a surge arrester actually work?

Surge arrester operating principle:

• Varistor (variable resistor) mechanism : Core component is metal oxide (zinc oxide + dopants) ceramic disc. Electrical resistance non-linear: highly dependent on applied voltage.
• Normal operating state : ~400V AC supply across arrester. Varistor impedance ~1 MΩ (mega-ohm; very high). Negligible current flows; arrester remains passive/transparent to circuit. No energy loss (ideal for continuous installation).
• Overvoltage surge detection : Lightning impulse or switching transient creates voltage spike (0.1–10 microseconds rise time, peak 5kV–100kV). Varistor detects overvoltage; crystal structure deforms microscopically.
• Impedance collapse : When voltage exceeds breakdown threshold (typically 600–1000V DC peak), varistor impedance drops 1000×: from 1MΩ to ~1Ω within nanoseconds. Acts like electronic gate opening.
• Surge current path : Now low-impedance path established from live circuit → varistor → earth lead → facility earth grid. Surge current diverted (kA magnitude typical for lightning); live voltage limited to ~1.5–2 kV during surge event.
• Energy dissipation : Surge energy dissipated as heat in varistor ceramic. Joule heating: I²R where R now low. Single-event energy capacity typically 10–100 joules per device. Multiple strikes may exceed capacity (arrester degrades/fails short).
• Recovery after surge : Surge transient passes (microseconds). Varistor returns to high impedance (1MΩ). Circuit voltage returns to 400V AC. Arrester transparent again; no residual current.
• Limitation voltage (Uc) : Arrester specification states maximum voltage during surge (e.g., "1500V peak limitation voltage"). Equipment protected if rated >1500V insulation.
• Lifetime degradation : Each surge event causes minor varistor crystal damage. Multiple surges degrade performance: leakage current increases, limitation voltage rises, eventual short-circuit failure. Typical arrester lifespan 10–15 years in moderate lightning zones.

Socomec surge arresters designed per IEC 61643-1, tested to IEC 61643-11 lightning impulse (10/350 μs waveform). Type 1 (10/350 μs high-current capable), Type 2 (8/20 μs standard), Type 3 (combination) ratings. Optim-Elec selects arrester discharge class per installation risk zone. Contact optim-elec.com.